Combustion dynamics occur for example in gas turbines at different places or chambers, where combustion dynamics occur. Examples for such places or chambers are the combustor chamber, a mixing chamber, a plenum and air channels. For reasons of simplification subsequently the term “chamber” is used and comprises all locations where combustion dynamics occur. In these chambers a gas (for example a mixture of fuel and air or exhaust gas) flows with high velocity which causes combustion dynamics. Burning air and fuel in the combustor chamber causes further combustion dynamics.
To reduce these combustion dynamics it is well known in the art, to install acoustic damping devices like a Helmholtz resonator, a half-wave tube, a quarter-wave tube or other types of damping devices with or without flow through of gas.
These acoustic damping devices have one or more resonance frequencies. If under operation of the gas turbine the combustions dynamics stimulate the resonance frequencies of the acoustic damping devices, the combustion dynamics are reduced or damped.
FIG. 1 illustrates the reflection coefficient (Y-Axis) and its dependency from the frequency.
The line 1 shows the theoretical reflection coefficient when using an acoustic damping device with a resonance frequency of approximately 300 Hertz. As can be seen at a frequency of 300 Hertz the reflection coefficient has a relative minimum of approximately 0.5. At frequencies of approximately 225 Hertz and 375 Hertz, the reflection coefficient has a local maximum of about 0.75.
To give an example: We assume that, for example, a combustion chamber of a gas turbine is equipped with an acoustic damping absorber having a resonance frequency of 300 Hertz. If we further assume that under operation in this combustion chamber noises ensue comprising frequencies of 300 Hertz it can be expected, that due to the local minimum of the reflection coefficient at 300 Hertz the noises with a frequency of 300 Hertz are effectively damped and therefore the noise emission of the combustor is reduced.
At an installed gas turbine the applicant has made some measurements and compared the theoretical reflection coefficient (line 1) with measurements taken at a same frequency range between 50 Hertz and 400 Hertz.
The measured values are illustrated in FIG. 1 by dots 3. By comparing the measured values with the theoretical reflection coefficient it can be seen that in the range between 250 Hertz and 350 Hertz the measured values 3 do not show a local minimum as should be expected.
In other words: The acoustic damping device does not work sufficiently.